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When a host approaches, four "petals" at the top of the egg spread open and a Facehugger launches out. The Engineer cargo hold used an anesthetic mist to put them in suspended animation for safe handling. When no host approaches, the egg develops several tendril like roots, probably for nutrient absorption similar to plant organisms, to help it sustain itself until a potential host appears. Another theory is that the tendrils are used for communication between eggs, like the synapses in brain cells, to inform other eggs of host approach.

Giger initially designed the eggs with a much more obviously vaginal appearance, complete with an "inner and outer vulva". The producers complained that Catholic countries would ban the film if the allusion was too strong, so Giger doubled the lobes to four, so that, in his words, "seen from above, they would form the cross that people in Catholic countries are so fond of looking at." The interior of the original egg was composed of "Nottingham lace", which is the lining of a cow's stomach. In the first film, the quick shot of the facehugger erupting from the egg was done with sheep's intestine. Initially the egg remained totally stationary save for the hydraulic movement of the lobes; however, by Alien Resurrection the entire egg was made to ripple as it opened.